National governments kill people. What separates the good ones from the rest is whether there's a sound process to determine who can be dispatched to the hereafter, and whether there's any way to hold the head of state who makes the call accountable.

Right now, it's not clear which camp the United States inhabits.

The White House has a kill list. President Barack Obama personally decides who is on it, including American citizens abroad suspected of terrorism. And some of those targeted have been killed, usually in drone strikes. That's an extraordinary amount of power for any president to wield without judicial review, public accountability or congressional oversight. Too much.

As Obama's deputy national security adviser for homeland security and counterterrorism for four years, Brennan has reportedly been one of the select few people in the room when those life-or-death decisions were made.

Members of the Senate committee should press him for details on the process the White House uses when weighing whether to place a name on the kill list. What criteria are considered? How is the evidence vetted? What's the standard of proof? Is the process more rigorous for American citizens? Is anyone outside the executive branch involved?

Sen. Ron Wyden (D-Ore.), has demanded that committee members be allowed to review secret Justice Department legal opinions justifying the targeted execution of American citizens. That's a start.

The nature of warfare has changed. When the enemy is a stateless organization, targeted strikes and special forces operations, like the one that killed Osama bin Laden, are more efficient and effective than sending in the troops. Targeted executions are widely credited for decapitating, dispersing and debilitating al-Qaida.

But Obama crossed a momentous line when he put American citizen and al-Qaida operative Anwar al-Awlaki on his hit list in 2009, and then executed him in 2011 when a drone fired hellfire missiles into his car in Yemen.

It may be a line that needed to be crossed. Al-Awlaki was an avowed terrorist with a hand in plotting two botched attacks: the Detroit underwear bombing and the Times Square bombing. But when an American president claims the power to execute U.S. citizens outside a war zone and without any judicial review of the evidence against them, that demands more scrutiny than it's been given.

The Senate should press Brennan on what rules he thinks are needed. To start, only people actively planning or participating in attacks should be targeted. Execution should be a last resort. When the subject is an American citizen, the evidence should be presented -- in secret if necessary -- to a judicial body such as the Foreign Intelligence Surveillance Court before the name can be added to the kill list. And Congress should provide aggressive oversight.

Protecting the homeland is any president's top job. But choosing who dies shouldn't be his decision alone.